System.Linq.Dynamic.Core : How do I make a select many correctly?

The project here with the documentation here looks to be the most current version of Dynamic LINQ.

Typically, you should only use Dynamic LINQ when the types are known at compile time rather to conventional LINQ.

When compiling, if you anticipate that the query will be against aList<Pack> standard LINQ, as seen in the following code, which updates the initial instances ofPack ):

var usefulPacks = Packs.Select(pack => {
    pack.Equipments = pack.Equipments.Where(equipment => 
        equipment.GenericEquipment.Id == 1
    ).ToList(); 
}).Where(pack => pack.Equipments.Any()).ToList();

This code makes duplicates of the original instances if you require code that doesn't change the original ones:

var usefulPacks = Packs.Select(pack => {
    return new Pack() {
        IDAtSource = pack.IDAtSource,
        Equipments = pack.Equipments.Where(equipment => 
            equipment.GenericEquipment.Id == 1
        ).ToList(); 
    };
}).Where(pack => pack.Equipments.Any()).ToList();

Keep in mind that this isn't utilizing.SelectMany — .SelectMany is used to combine many enumerables into one; in this case, eachPack the last item on the list refers to aPack originally on the list.

Modifying or initializing properties as part of an expression is not supported by dynamic LINQ:

The expression language permits getting (but not setting) the value of any reachable public field, property, or indexer.

so theEquipments attribute cannot be altered or set to only include instances ofEquipment that meet the requirements.

To determine theEquipments you have two options:

1. Include a constructor inPack which considers the relevant arguments
2. Create a static method for every class that accepts the necessary parameters.

Include a constructor inPack

You might include a constructor inPack With the proper justifications, that establishesEquipments :

Pack(int IDAtSource, IEnumerable<Equipment> equipments) {
    this.IDAtSource = IDAtSource;
    this.Equipments = equipments.ToList();
}

then you might use the subsequent:

IQueryable qry = Packs.AsQueryable();
qry = qry
    .Select("Pack(IDAtSource, Equipments.Where(GenericEquipment.ID=1))")
    .Where("Equipments.Any");

Establish a static method

public static class MyPackMethods {
    public static Pack Create(int IDAtSource, IEnumerable<Equipment> equipments) {
        return new Pack() {
            IDAtSource = IDAtSource,
            Equipments = equipments.ToList()
        };
    }
}

then dial:

IQueryable qry = Packs.AsQueryable();
qry = qry
    .Select("MyPackMethods.Create(IDAtSource, Equipments.Where(GenericEquipment.ID=1))")
    .Where("Equipments.Any");